Ink, ink-jet recording method using the same, and photopolymerization initiator

ABSTRACT

An ink for ink-jet recording contains a coloring agent, a polymerizable oligomer, water, and a photopolymerization initiator having a solubility in water of 3 percent by weight or more. Another ink for ink-jet recording contains a coloring agent, a polymerizable oligomer having at least two acryloyl groups and a solubility in water of 10 percent by weight or more, a photopolymerization initiator, and water. The specified polymerizable oligomer or photopolymerization initiator reduces bleeding of the ink on recording media.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a UV-curable ink preferably used inink-jet recording, an ink-jet recording method improving image quality,and a novel photopolymerization initiator having high solubility inwater.

2. Description of the Related Art

Examples of aqueous ink-jet inks in use include an aqueous black dye inkcontaining a water-soluble black dye dissolved in an aqueous medium, andan aqueous black pigment ink containing a black pigment such, as carbonblack, in an aqueous medium.

Recording using an aqueous black dye ink facilitates a shortened initialfixing time, yields a satisfactory image density, and causes little orno bleeding between this ink and color inks. The recorded image,however, does not generally have sufficient water resistance, smearresistance, and light durability. When the recorded image is coveredwith a marker pen (high-lighter), the recorded characters may blur. Inparticular, in documents including black characters and color images,mutual bleeding may occur between a black character and the adjacentcolor images and results in a lack of crispness of the characters. Sucha problem requires appropriate solutions, for example, a reducedprinting rate.

An aqueous black pigment ink tends to require a prolonged initial timefor fixing the pigment to a recording medium, which inhibits improvementin printing rate. Furthermore, increased printing density causesdecreased smear or scuff resistance; hence, ink transferred by scuffmarks may cause contamination, or tracing of the printed section with acolor marker pen may lessen the crispness of the printed characters.Aqueous black pigment inks in practical use readily cause bleeding withcolor inks; hence, the use of such inks may be restricted in documentscontaining characters and color images in some cases.

Various means have been disclosed for solving such technical problems.Examples of improvement in smear resistance of aqueous pigment inksinclude addition of water-soluble resins as binding resins in inks,addition of curable resins, and addition of microemulsion particles. Ameans for reducing the fixing time includes use of a solvent whichaccelerates the penetration of the ink into the recording medium, thesolvent being capable of suppressing bleeding between black and colorsites and suppressing the decrease in optical density of the characters.A method for avoiding color bleeding includes use of a black inkcontaining a cationic polymer and carbon black which suppresses mutualdiffusion with an anionic color ink.

Although these methods can effectively improve individually targetedproblems, they are insufficient overall improvement.

Furthermore, one of the typical means for improving the smear resistanceof a printed image using aqueous inks includes use of a UV-curable ink.UV-curable inks have been used for marking in industrial apparatuses inknown ink-jet recording technologies. These inks are nonaqueous inksusing ketone solvents. UV-curable inks are also disclosed in JapanesePatent Publication No. 5-64667, Japanese Patent Application Laid-OpenNos. 5-186725, and 7-224241, and U.S. Pat. No. 4,978,969.

An ink disclosed in Japanese Patent Publication No. 5-64667 contains apigment and a polymerizable polyacrylate having a functionality of threeor more as essential components, and a ketone and an alcohol as primarysolvents. An ink disclosed in Japanese Patent Application Laid-Open No.5-186725 contains water and a photo-curable prepolymer and is used inink-jet printers. An ink disclosed in Japanese Patent ApplicationLaid-Open No. 7-224241 is an aqueous UV-curable pigment ink used inbubble jet processes. This ink contains a pigment, an aqueous polymericcompound for dispersing the pigment in an aqueous medium, awater-soluble monomer, an aqueous medium, and a polymerizationinitiator, and has a viscosity of 10 cps or less. U.S. Pat. No.4,978,969 discloses a Fv method for forming an image by discharging anink containing a UV-curable adhesive by a thermal ink-jet process.

SUMMARY OF THE INVENTION

The present inventors have further studied conventional photo-curableinks for ink-jet recording, taking into consideration that an ink-jetrecording process using a photo-curable ink is a method that possiblycan improve the above-mentioned problems totally. Conventionalphoto-curable inks generally contain polymerizable oligomers andphotopolymerization initiators which are not at all soluble or areslightly soluble in aqueous media, and contain various water-solubleorganic solvents to stabilize discharge in ink-jet recording. However,such organic solvents may affect the penetrability of the inks intorecording media, and thus affect image quality. The present inventorshave concluded that such organic solvents are one of the reasons whyconventional photo-curable inks are not sufficient to improve theaforementioned problems totally.

Accordingly, it is an object of the present invention to provide aphoto-curable ink which does not substantially affect ink-jet recordingperformance even when a polymerizable oligomer and a photo-initiator arecontained in the ink as essential components.

It is another object of the present invention to provide an ink-jetrecording method which facilitates formation of a high-quality imageusing a photo-curable ink.

It is still another object of the present invention to provide anink-jet recording method which facilitates formation of a multicolorimage using a photo-curable ink.

According to a first aspect of the present invention, there is providedan ink for ink-jet recording comprising a coloring agent, apolymerizable oligomer, a photopolymerization initiator and water, thephotopolymerization initiator having a solubility in water of 3 percentby weight or more.

According to a further aspect of the present invention, there isprovided an ink for ink-jet recording comprising a coloring agent, apolymerizable oligomer, a photopolymerization initiator and water, thepolymerizable oligomer having at least two acryloyl groups and asolubility in water of 10 percent by weight or more.

According to a further aspect of the present invention, there isprovided a method for forming an image on a recording medium comprisingthe steps of: (a) imparting an ink comprising a coloring agent, apolymerizable oligomer, a photopolymerization initiator having asolubility in water of 3 percent by weight or more, and water, on thesurface of the recording medium; and (b) irradiating the ink on therecording medium with light.

According to a further aspect of the present invention, there isprovided a method for forming an image on a recording medium comprisingthe steps of: (a) imparting an ink for ink-jet recording on therecording medium, the ink comprising a coloring agent, a polymerizableoligomer having at least two acryloyl groups and a solubility in waterof 10 percent by weight or more, a photopolymerization initiator, andwater, and (b) irradiating the ink on the recording medium with light.

In these aspects, the oligomer and the photopolymerization initiator ineach photo-curable ink have high solubility in water; hence, addition ofan organic solvent, which adversely affects the image quality, fordissolving these components in the ink is not substantially required. Asa result, a high-quality image is obtainable.

According to a further aspect of the present invention, there isprovided a photopolymerization initiator represented by the followinggeneral formula (1):

wherein n is an integer of 1 or more, m is an integer of 0 or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.

Since the photopolymerization initiator effectively forms radicals byultraviolet light and has high solubility in water compared toconventional nonionic photopolymerization initiators, it can bepreferably used for aqueous photo-curable ink-jet inks and aqueousphoto-curable paints.

According to a still further aspect of the present invention, there isprovided a method for recording a multicolor image including a blackimage by imparting a black ink and at least one color ink selected froma yellow ink, a magenta ink, and a cyan ink to a recording medium by anink-jet process, comprising the steps of:

(a) imparting a black ink having any one of the following Compositions(i) to (iv) to the recording medium by an ink-jet process:

Composition (i)

at least one coloring agent selected from the group consisting of acombination of a black pigment and a water-soluble cationic polymericcompound for dispersing the black pigment in water, a cationic blackdye, and a cationic self-dispersing carbon black, a nonionic or cationicpolymerizable oligomer, a nonionic or cationic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more,and water;

Composition (ii)

at least one coloring agent selected from the group consisting of acombination of a black pigment and a water-soluble cationic polymericcompound for dispersing the black pigment in water, a cationic blackdye, and a cationic self-dispersing carbon black, a nonionic or cationicpolymerizable oligomer having at least two acryloyl groups and having asolubility in water of 10 percent by weight or more, a nonionic orcationic photopolymerization initiator, and water;

Composition (iii)

at least one coloring agent selected from the group consisting of acombination of a black pigment and a water-soluble anionic polymericcompound for dispersing the black pigment in water, an anionic blackdye, and an anionic self-dispersing carbon black, a nonionicpolymerizable oligomer, a nonionic photopolymerization initiator havinga solubility in water of 3 percent by weight or more, and water;

Composition (iv)

at least one coloring agent selected from the group consisting of acombination of a black pigment and a water-soluble anionic polymericcompound for dispersing the black pigment in water, an anionic blackdye, and an anionic self-dispersing carbon black, a nonionicpolymerizable oligomer having at least two acryloyl groups and having asolubility in water of 10 percent by weight or more, a nonionicphotopolymerization initiator, and water;

(b) imparting a color ink having any one of the following Compositions(v) to (viii) to the recording medium by an ink-jet process:

Composition (v)

at least one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble cationic polymericcompound for dispersing the color pigment in water, and a cationic colordye, a nonionic or cationic polymerizable oligomer, a nonionic orcationic photopolymerization initiator having a solubility in water of 3percent by weight or more, and water;

Composition (vi)

at least one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble cationic polymericcompound for dispersing the color pigment in water, and a cationic colordye, a nonionic or cationic polymerizable oligomer having at least twoacryloyl groups and having a solubility in water of 10 percent by weightor more, a nonionic or cationic photopolymerization initiator, andwater;

Composition (vii)

at least one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble anionic polymericcompound for dispersing the color pigment in water, and an anionic colordye, a nonionic polymerizable oligomer, a nonionic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more,and water;

Composition (viii)

at least one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble anionic polymericcompound for dispersing the color pigment in water, and an anionic colordye, a nonionic polymerizable oligomer having at least two acryloylgroups and having a solubility in water of 10 percent by weight or more,a nonionic photopolymerization initiator, and water; and

(c) irradiating the black ink and the color ink imparted to therecording medium with light for photo-crosslinking of the inks, whereinthe black ink and the color ink are selected so that the polarity of thecoloring agent and the water soluble polymeric compound of the black inkare opposite to the polarity of the coloring agent and the water solublepolymeric compound of the color ink.

The method according to this aspect can effectively suppress bleedingbetween adjacent different color inks on the recording medium beforephoto-crosslinking, resulting in formation of a high-quality image.

Further objects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a configuration of an ink-jet printerusable in the present invention;

FIG. 2 is a longitudinal cross-sectional view of an embodiment of a headof an ink-jet recording apparatus;

FIG. 3 is a cross-sectional view taken along line III—III in FIG. 2;

FIG. 4 is an outline view of a multihead;

FIG. 5 is an isometric outline view of an embodiment of an ink-jetrecording apparatus;

FIG. 6 is a longitudinal cross-sectional view of an embodiment of an inkcartridge;

FIG. 7 is an isometric view of an embodiment of a recording unit;

FIG. 8 is an isometric outline view of another configuration of anink-jet recording head;

FIG. 9 is an isometric outline view of a recording head provided withfour ink cartridges;

FIG. 10 is an isometric outline view of a configuration of fourrecording heads arranged on a carriage;

FIG. 11 is a liquid chromatogram of Irgacure 2959; and

FIG. 12 is a liquid chromatogram of reaction products of Irgacure 2959with ethylene oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An ink for ink-jet recording according to an embodiment of the presentinvention is a photo-curable ink and comprises a coloring agent, apolymerizable oligomer, a photopolymerization initiator having asolubility in water of 3 percent by weight or more, and water.

Ink Ingredients

A. Coloring Agent

Various coloring agents can be used in such an ink for ink-jetrecording. Examples of coloring agents include pigments, anionic dyes,and cationic dyes. Examples of pigments include self-dispersing pigmentshaving surface cationic or anionic groups. Pigments that are notdissolved in aqueous media but are stably dispersed in aqueous media bythe effects of water-soluble cationic polymeric compounds orwater-soluble anionic polymeric compounds, can also be used as thecoloring agents of the present invention in combination with thewater-soluble polymeric compounds. By selecting the coloring agent,either a black ink or a color ink is prepared in the present invention.

A1. Black Ink

Coloring agents for black ink will now be described.

A1-1. Black Pigment

Carbon Black as Black Pigment

(a) Cationic Self-Dispersing Black Pigment

A cationic self-dispersing black pigment may be a black pigment havingcationic groups or may be a dispersion of a black pigment in awater-soluble polymer having cationic groups. In the latter case, thecationic water-soluble polymer functions as a dispersant for the blackpigment particles and reacts with an anionic dye or dispersant in anaqueous color ink. Such reaction can suppress color bleeding inmulticolor printing caused by mutual diffusion between adjacent dots.

In the cationic self-dispersing black pigment having cationic groups,the cationic groups are covalently bonded to the surface of theself-dispersing black pigment so that the pigment can disperse in an inkwithout dispersion treatment. Examples of cationic groups include anammonium cation, a quaternary ammonium salt of a tertiary amine, apyridinium cation, and a sulfonium cation. The cationic groups on thepigment surface may consist of the same type or different types. Thecationic self-dispersing black pigment imparts desired characteristicsto an aqueous black ink in an acidic or neutral pH region.

Preferable cationic self-dispersing carbon black is prepared by a methoddisclosed in, for example, “The 50th Imaging Science & Technologies” byJ. E. Johnson (Cabot Co.), p. 310, and U.S. Pat. No. 5,554,739 (CabotCo.). The cationic carbon black is a surface-modified pigment havingcationic groups, such as a quaternary ammonium [R₄N]⁺ group, aquaternary phosphonium [R₄P]⁺ group, an N-methylpyridinium group, and anN-phenylpyridinium group. A carbon black pigment having a hydrochloricacid salt of a tertiary amine can also be used in the present invention.Such a carbon black pigment is prepared by the same method, that is,unsaturated double bonds on the pigment are allowed to react with acompound which has a tertiary amino group at one end and an aromaticdiazonium salt at the other end so that atomic groups having tertiarynitrogen atoms are bonded to the pigment surface, and the salt isneutralized with hydrochloric acid. Another method for preparing acationic carbon black pigment is bonding of a lower polymer or oligomerby graft polymerization of an acrylic monomer having a cationic sidechain onto the surface of the black pigment. The resulting cationiccarbon black pigment may also be used in the present invention.

The average particle size or diameter of the black pigment, such ascarbon black, having surface cationic groups is in the range of 70 nm to250 nm. The black pigment having surface cationic groups can effectivelysuppress mutual diffusion between a cationic ink containing the blackpigment and an anionic ink.

(b) Carbon Black+Water-soluble Cationic Polymeric Compound

Addition of a water-soluble cationic polymeric compound as a dispersantdisclosed in Japanese Patent Application Laid-Open No. 8-80665 enablesuse of a general carbon black in an ink. That is, a dispersion of blackpigment particles dispersed with a water-soluble cationic polymericcompound is also preferably used in the present invention. Examples ofwater-soluble cationic polymeric compounds include copolymers of thefollowing monomers; N,N-dimethylaminoethyl methacrylate,N,N-dimethylaminoethyl acrylate, N,N-dimethylaminopropyl methacrylate,N,N-dimethylaminopropyl acrylate, N,N-dimethylacrylamide,N,N-dimethylmethacrylamide, N,N-dimethylaminoethylacrylamide,N,N-dimethylethylmethacrylamide, N,N-dimethylaminopropylacrylamide, andN,N-dimethylaminopropylmethacrylamide. The copolymer contains themonomer in an amount of 10 to 50 percent by weight. Examples of thecomonomers in these copolymers include water-soluble monomers, such asacrylamide, acrylonitrile, and alkyl acrylates.

The average particle size or diameter of the black pigment, such ascarbon black, dispersed with a water-soluble cationic polymeric compoundis in the range of 70 nm to 250 nm. Examples of the cationic groups inthe water-soluble cationic polymeric compound include an ammonium groupand a quaternary ammonium salt of a tertiary amine. Examples of sourcesof counter ions to the cationic groups include acetic acid, lactic acid,hydrochloric acid, sulfuric acid, and succinic acid. The counter ionsstabilize dissociation and thus enhance dispersibility in the aqueousmedium.

The amount of the cationic groups imparted to the black pigment orwater-soluble polymeric compound may be adequately determined as long assatisfactory dispersion in the ink and advantages of the presentinvention are achieved.

A black pigment in an amount of 0.5 to 10 parts by weight is added to100 parts by weight of ink. The pH of the ink is in a range of 2.5 to8.0. A combination of a pH in the above range and a particle size in theabove range is preferable to further stabilize dispersion of the ink.

(c) Anionic Self-Dispersing Carbon Black

In a typical anionic self-dispersing carbon black, anionic groups arecovalently bonded to the surface of carbon black. Examples of anionicgroups include —COOM, —SO₃M, —PO₃HM, and —PO₃M₂, wherein M is a hydrogenatom, an alkaline metal, an ammonium ion, or an organic ammonium group.

Negatively charged carbon black having —COOM or —SO₃M groups directly orindirectly bonded to the surface imparts superior dispersibility to inkand is thus preferably used. Examples of the alkaline metals (M) includelithium, sodium and potassium. Examples of organic ammonium groupsinclude monomethylammonium to trimethylammonium, andmonomethanolammonium to triethanolammonium. A method for making theanionic self-dispersing carbon black is oxidation of carbon black withsodium hypochlorite. By this method, —COONa groups are bonded to thesurface of the carbon black.

The above-mentioned hydrophilic groups may be directly bonded to thesurface of carbon black. Alternatively, other atomic groups mayintervene between the surface and the hydrophilic groups to introduceindirect bonding between the hydrophilic groups and the surface.Examples of the other atomic groups include linear or branched alkylenegroups having 1 to 12 carbon atoms, substituted or unsubstitutedphenylene groups, and substituted or unsubstituted naphthylene groups.Examples of the substituent groups in the substituted phenylene groupsand the substituted naphthylene groups include linear or branched alkylgroups having 1 to 6 carbon atoms. Examples of combination of the otheratomic groups with the hydrophilic groups include —C₂H₄—COOM, —Ph—SO₃M,and —Ph—COOM, wherein Ph is a phenylene group.

(d) Carbon Black+Water-soluble Anionic Polymeric Compound

In the black pigment of the present invention, a general carbon blackmay be used with a water-soluble anionic polymeric compound as adispersant. Examples of water-soluble anionic polymeric compoundsinclude polymers soluble in alkaline aqueous solution, e.g.,styrene-acrylic acid copolymers, styrene-acrylic acid-alkyl acrylatecopolymers, styrene-maleic acid copolymers, and styrene-maleicacid-acrylic acid copolymers.

A1-2. Black Dye

Black dyes are also used as black coloring agents. Since the ink inaccordance with the present invention is cured by ultraviolet light on arecording medium, use of a dye having good light resistance ispreferable. Examples of preferable black dyes include monoazo or bisazocomplexes having multivalent metallic ligands, such as Cr, Cu, Mn, Al,Zn, and Fe, and noncomplex-type azo-black dyes, such as C.I. DirectBlack 17, C.I. Direct Black 19, C.I. Direct Black 51, C.I. Direct Black154, C.I. Direct Black 174, and C.I. Direct Black 195.

Black cationic dyes can also be used as coloring agents. Examples of theblack cationic dyes include Aizen Cationic Black SBH, Aizen CationicBlack BXH, Aizen Cationic Black SH, XAizen Cationic Black ACH, AizenCationic Black MH, and Aizen Cationic Black TH made by Hodogaya ChemicalCo., Ltd.; Sumiacryl Black B, Sumiacryl Black R, Sumiacryl Black AP,Sumiacryl Black BP, Sumiacryl Black CP, Sumiacryl Black PPP made bySumitomo Chemical Co., Ltd.; and Diacryl Supra Black GSL, Diacryl SupraBlack RSL, and Diacryl Supra Black ESL made by Mitsubishi ChemicalCorporation.

A2. Color Ink

Color inks used in color recording will now be described.

A2-1. Aqueous Color Ink

An aqueous color ink used in the present invention may be an aqueousanionic dye dissolved in an aqueous medium or an anionic organic pigmentdispersed in an aqueous dispersant. The aqueous color ink may be of anultraviolet curable type, if necessary. Dyes or pigments which arestable against light such as ultraviolet light are preferably used ascoloring agents in an ultraviolet-curable-type aqueous color ink. Thedose of ultraviolet light for practical curing treatment is generallynegligible with respect to discoloration of dyes and pigments that havetypically been used. Such low energy irradiation permits use of knowndirect dyes, acidic dyes, reactive dyes, metal complexes thereof, andmultivalent metal salts thereof.

A2-2. Color Pigment

Examples of organic pigments which are used in color inks and have hueas process color are as follows:

(A) Yellow pigments: Pigment Yellow 1, Pigment Yellow 2, Pigment Yellow3, Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, PigmentYellow 16, Pigment Yellow 17, Pigment Yellow 55, Pigment Yellow 73,Pigment Yellow 74, Pigment Yellow 75, Pigment Yellow 83, Pigment Yellow93, Pigment Yellow 951 Pigment Yellow 97, Pigment Yellow 98, PigmentYellow 109, Pigment Yellow 110, Pigment Yellow 114, Pigment Yellow 128,Pigment Yellow 138, Pigment Yellow 139, Pigment Yellow 150, PigmentYellow 151, Pigment Yellow 154, and Pigment Yellow 180.

(B) Magenta pigments: Pigment Red 5, Pigment Red 7, Pigment Red 12,Pigment Red 48(Ca), Pigment Red 48(Mn), Pigment Red 57:1, Pigment Red57(Sr), Pigment Red 57:2, Pigment Red 122, Pigment Red 123, Pigment Red168, Pigment Red 184, and Pigment Red 202.

(C) Cyan pigments: Pigment Blue 1, Pigment Blue 2, Pigment Blue 3,Pigment Blue 16, Pigment Blue 22, Pigment Blue 60, Pigment Blue 15:2,Pigment Blue 15:3, Vat Blue 4, and Vat Blue 60.

A2-3. Anionic Color Dye

Although dyes are generally inferior to pigments in light resistance,dyes can be practically used by optimizing the wavelength and intensityof light for curing so that discoloration is reduced as much aspossible. Dyes to be used are determined depending on the application.Examples of anionic color dyes are as follows.

(A) Yellow dyes: Acid Yellow 11, Acid Yellow 17, Acid Yellow 23, AcidYellow 25, Acid Yellow 29, Acid Yellow 42, Acid Yellow 49, Acid Yellow61, Acid Yellow 71, Direct Yellow 12, Direct Yellow 24, Direct Yellow26, Direct Yellow 44, Direct Yellow 86, Direct Yellow 87, Direct Yellow98, Direct Yellow 100, Direct Yellow 130, Direct Yellow 132, DirectYellow 142, and dyes represented by the following formulae (34) and(35):

wherein R=H or CH₃, X=H, OH or CH₃, and n=1 or 2;

wherein R=H or CH₃, X=H, OH or CH₃, —L—=—CH₂CH₂—,

and n=1 or 2.

(B) Magenta dyes: Acid Red 1, Acid Red 6, Acid Red 8, Acid Red 32, AcidRed 35, Acid Red 37, Acid Red 51, Acid Red 52, Acid Red 80, Acid Red 85,Acid Red 87, Acid Red 92, Acid Red 94, Acid Red 115, Acid Red 180, AcidRed 254, Acid Red 256, Acid Red 289, Acid Red 315, Acid Red 317, DirectRed 1, Direct Red 4, Direct Red 13, Direct Red 17, Direct Red 23, DirectRed 28, Direct Red 31, Direct Red 62, Direct Red 79, Direct Red 81,Direct Red 83, Direct Red 89, Direct Red 227, Direct Red 240, Direct Red242, Direct Red 243 and dyes represented by the following formulae (36)to (38):

wherein R=H, OH or CH₃, —L—=—CH₂CH₂—,

and n=1 or 2;

wherein R=OH or CH₃, and n=1 or 2.

(C) Cyan dyes: Acid Blue 9, Acid Blue 22, Acid Blue 40, Acid Blue 59,Acid Blue 93, Acid Blue 102, Acid Blue 104, Acid Blue 113, Acid Blue117, Acid Blue 120, Acid Blue 167, Acid Blue 229, Acid Blue 234, AcidBlue 254, Direct Blue 6, Direct Blue 22, Direct Blue 25, Direct Blue 71,Direct Blue 78, Direct Blue 86, Direct Blue 90, Direct Blue 106, DirectBlue 199, and dyes represented by the following compounds (39) and (40):

wherein x+y 3 or 4, X=H, NH₂ or CH₃, R=H or CH₂, —L—=—CH₂CH₂—,

and n=1 or 2;

wherein x+y=3 or 4.

In addition to these known coloring agents, newly developed compoundscan also be used in the present invention without limitation, as long asthe compounds satisfy hue, light resistance, and solubilityrequirements.

A2-4. Cationic Color Dye

Cationic dyes generally have clear hue and good coloringcharacteristics, although they are slightly inferior to pigments inlight resistance. Thus, these dyes can also be used in the presentinvention by selecting the type of light and by optimizing theirradiation time. Cationic dyes have basic structures, such as cyanine,azo, azomethine, xanthene, triphenylmethane, methane, polymethine, andphthalocyanine. Examples of cationic color dyes include C.I. BasicYellow 1, C.I. Basic Yellow 11, C.I. Basic Yellow 13, C.I. Basic Yellow19, C.I. Basic Yellow 21, C.I. Basic Yellow 25, C.I. Basic Yellow 33,C.I. Basic Yellow 36, C.I. Basic Red 1, C.I. Basic Red 2, C.I. Basic Red9, C.I. Basic Red 12, C.I. Basic Red 13, C.I. Basic Red 38, C.I. BasicRed 39, C.I. Basic Red 92, C.I. Basic Blue 1, C.I. Basic Blue 3, C.I.Basic Blue 5, C.I. Basic Blue 9, C.I. Basic Blue 19, C.I. Basic Blue 24,C.I. Basic Blue 25, C.I. Basic Blue 26, C.I. Basic Blue 28, C.I. BasicBlue 45, C.I. Basic Blue 54, and C.I. Basic Blue 65. Variouscombinations of these cationic color dyes may produce a cationic blackdye.

B. Polymerizable Oligomer

As described above, preferable polymerizable oligomers have two or moreacryloyl groups in each molecule and have a solubility in water of 10percent by weight or more. The polymerizable oligomer, which isgenerally present in a liquid state, has fundamental characteristics,such as being hydrophilic, highly hygroscopic, and nonvolatile, as asolvent component in an aqueous ink-jet ink. The most important propertyof the polymerizable oligomer is rapid polymerization by ultraviolet rayirradiation after printing. The oligomer is solidified by polymerizationto fix the coloring agent on a recording medium such as paper. The rapidpolymerization can effectively prevent undesirable mixing with othercolor inks and contamination of the output recording medium; hence, theprinting rate can be increased.

Examples of polymerizable oligomers used in the present inventioninclude acrylic or methacrylic esters of multivalent alcohols andacrylic or methacrylic esters of glycidyl ethers derived from themultivalent alcohols. The multivalent alcohols may be chain-extended byaddition of ethylene oxide. These aqueous nonionic polymerizableoligomers (not having dissociating groups) can be used for preparationof both cationic inks and anionic inks.

Examples of the nonionic oligomers are represented by the followingformulae (2) to (5):

The solubility in water of each nonionic oligomer is shown in thefollowing table.

Solubility in Water (%) Nonionic Oligomer 1 >20 Nonionic Oligomer 2 >20Nonionic Oligomer 3 15 Nonionic Oligomer 4 >20

In addition to these polymerizable oligomers, nonionic acrylic esterswhich are derived from polyhydroxyl compounds and have two or moreacryloyl groups in each molecule are also used in the present invention.Examples of the polyhydroxyl compound include condensation products ofglycols, oligoethers, and oligoesters.

Further examples of preferable nonionic water-soluble polymerizableoligomers include acrylic or methacrylic esters of polyols having atleast two hydroxyl groups, such as monosaccaride and disaccharide; andacrylic or methacrylic esters of triethanolamine, diethanolamine,trishydroxyaminomethane or trishydroxyaminoethane. These oligomers alsohave fundamental characteristics essential for aqueous ink in accordancewith the present invention.

A preferable aqueous cationic polymerizable oligomer has a basicstructure selected from the above-mentioned nonionic oligomers and hasat least one type of cationic group. An aqueous solvent in an aqueousink may be partly or completely replaced with the cationic polymerizableoligomer to reduce or eliminate the solvent in the ink. The cationicpolymerizable oligomer further enhances cationic characteristics of anaqueous black ink, and thus effectively prevents color mixing when theblack ink comes into contact with an anionic ink during printing.

Further examples of basic structures of polymerizable oligomers havingcationic groups are represented by the following formulae (6) to (17):

wherein R is a polyol moiety, X is hydrogen or CH₃, and A⁻ is Cl⁻, HSO₃⁻ or CH₃COO⁻.

These polymerizable oligomers also have a solubility in water of 10% ormore. Examples of polyol sources include glycerin, 1,2,4-butanetriol,1,2,5-pentanetriol, 1,2,6-hexanetriol, trimethylolpropane,trimethylolmethane, trimethylolethane, pentaerythritol, bisphenol A,alicyclic bisphenol A, and condensation products thereof.

Examples of polymerizable oligomers having cationic groups arerepresented by the following formulae (18) to (28):

wherein n+m+k=15;

The above polymerizable oligomers having high polymerization rates showgood adhesiveness on recording media and good miscibility in ink, andachieve good transparency in the image without causing a change in hue.Polymerizable oligomers having cationic groups show better solubility inwater due to enhanced hygroscopic properties. Since the cationicoligomers react with nonionic dyes or dispersants in aqueous color inksto form salts or to coagulate, they can effectively suppress colorbleeding and can be rapidly bonded to recording media. Accordingly, useof the polymerizable oligomers achieves satisfactory printing.

These polymerizable oligomers can be used in aqueous black inks andaqueous color inks. Examples of other polymerizable oligomers preferablyused in color inks also include acrylates, such as DENACOL (trade name,manufactured by Nagase Chemical Ltd.), and acrylic esters ofwater-soluble epoxy resins, such as DENACOL EX 512 and DENACOL DA931.

The polymerizable oligomer is added in an amount of 1 to 30 parts byweight to 100 parts by weight of ink, regardless of the type of thecoloring agent, that is, a pigment or a dye, and the type of the ink,that is, a cationic ink or an anionic ink. A preferable amount of thepolymerizable oligomer is in a range of 5 to 20 parts by weight,although the amount is determined in view of the amount of the coloringagent, and viscosity of the polymerizable oligomers. The preferablerange further improves adhesiveness of the coloring agent on a recordingmedium, water resistance of the printed image, transparency of thepolymer itself, curing characteristics during polymerization,preservation stability of the ink, and physical properties of the curedink.

A water-soluble cationic polymerizable oligomer having at least twoacryloyl groups, which is used with a black cationic pigment dispersionto produce a black ink, has a high solubility in water of 10 percent byweight or more, and has high moisture retention since it has a cationicgroup.

C. Photopolymerization Initiator

The photopolymerization initiator in accordance with the presentinvention has a solubility in water of 3 percent by weight or more, andinitiates polymerization of a polymerizable oligomer contained in an inkto form a polymer containing a coloring agent, such as a black pigment,as an inclusion so that the coloring agent is fixed to a recordingmedium.

Any type of photopolymerization initiator can be used without limitationin the present invention, as long as the initiators form radicals bylight irradiation, have a solubility in water of 3 percent or more, anddo not react with other components in the ink, such as the coloringagent.

C1. Nonionic Photopolymerization Initiator

Nonionic photopolymerization initiators can be used in inks containinganionic dyes or cationic dyes, inks containing pigment coloring agentsdispersed in anionic or cationic dispersants, and inks containingself-dispersing carbon black as a coloring agent having surface anionicor cationic groups. A nonionic photopdlymerization initiator having asolubility in water of 3 percent or more may be selected from knownnonionic photopolymerization initiators or may be prepared by modifyinga known initiator to improve its solubility. Examples of such nonionicphotopolymerization initiators include compounds having hydroxyl groups,e.g., 1-hydroxycyclohexyl phenyl ketone (trade name: Irgacure 184, madeby Ciba Specialty Co.), 2-hydroxy-2-methyl-1-phenyl-propan-1-one (tradename: Darocur 1173, made by Ciba Specialty Co.), and4-(2-hydroxyethyoxy)-phenyl-(2-hydroxy-2-methylpropyl) ketone (tradename: Irgacure 2959, made by Ciba Specialty Co.); and modified compoundsthereof that were modified to improve solubility in water. Anethylene-oxide-added compound which is prepared by modification ofIrgacure 2959 and is represented by the following formula (1) has asolubility in water of 3 percent or more, and substantially 20 percentor more, and is highly soluble in an aqueous 50%-glycerin solution;hence, this photopolymerization initiator is preferably used in thepresent invention:

wherein n is an integer of 1 or more, m is an integer of 0 or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.

When n is zero, the resulting initiator does not have sufficientsolubility in water. When the sum (x) of n and m is 1 and when n is 1and m is zero, the ink does not have satisfactory preservation stabilityor satisfactory discharging characteristics in practical use, regardlessof increased solubility. When the sum is 9 or more, the resultingphotopolymerization initiator has a large molecular size and causeseconomical disadvantages due to an increase in the content in ink,although the activity of the initiator does not decrease.

A structure which is essential for radical cleavage for ultravioletcuring is an atomic group having a hydroxyl group or an atomic groupequivalent to the hydroxyl group at the α-position. Between thefollowing processes, the second process is preferable, because it isdifficult to control the number of “m” and “n” in the first process. Itis clarified in the present invention that substitution of theα-hydroxyl group by an ether does not inhibit radical formation, asshown in the above formulae. Addition of ethylene oxide in an amount upto eight equivalents is reasonable in practice. The photopolymerizationinitiator represented by the formula (1) does not have an ionizinggroup, and is preferably used together with a water-soluble anionic dyeor an aqueous pigment dispersion.

This compound is prepared by the following processes.

1. First Process (Addition of Ethylene Glycol Group)

wherein x=n+m;

2. Second Process (Addition of Hydroxyl Group)

Examples of photopolymerization initiators are represented by thefollowing formulae (29) to (33):

The following table shows the solubility of each compound.

Solubility in Solubility in Water Water/Glycerin (50:50) Compound 294% >10% Compound 30 >10% >20% Compound 31 >20% >20% Compound32 >20% >20% Compound 33 >20% >20%

A water-soluble solvent, which can dissolve the above-mentionedcompounds and is miscible with water, may be used as a medium of anaqueous ink. Examples of water-soluble solvents include benzoin ethers,e.g., benzoin methyl ether, benzoin ethyl ether, and benzoin isopropylether; acryloxime esters; dialkoxyacetophenones, e.g.,p-dimethylaminoacetophenone and 2,2-diethoxyacetophenone;chloroacetophenones, e.g., 2-chlorothioxanthone; benzylketals, e.g.,benzyldimethylketal; benzophenone; and mixtures thereof withhydrogen-donating aryl amines.

The above-mentioned Irgacure 184 and Darocur 1173, each having onehydroxyl group per molecule, may have two to ten ethylene oxide chainsin each molecule. Such initiators further improve solubility in water.

C2. Cationic Photopolymerization Initiator

When the ink contains a coloring agent, such as a cationic dye, apigment dispersed in a cationic dispersant, or a self-dispersing carbonblack having surface cationic groups, a cationic photopolymerizationinitiator can also be used, as well as the above nonionicphotopolymerization initiator. When a cationic pigment or dye having asolubility in water of 3% or more is used, a cationicphotopolymerization initiator having a solubility in water of 30% ormore can be used. Examples of such a compound are represented by thefollowing formulae:

(1) 4-benzoyl-N,N,N-trimethylbenzen methane ammonium chloride

(2) 2-hydroxy-3-(4-benzoyl-phenoxy)-N,N,N-trimethyl-1-propane ammoniumchloride

(3) 4-benzoyl-N,N-dimethyl N-[2-(1-oxo-2-propenyloxy)ethyl]benzenemethane ammonium chloride

Other hydrophilic polymerization catalysts preferably used in thepresent invention are disclosed by Manfred Kohler (Ciba Specialty Co.)in European Coatings Journal, December 1997, pp. 1118-1120, “A versatileα-hydroxyketone photoinitiator”.

The content of the photopolymerization initiator in the ink forachieving a satisfactory curing rate is in the range of 0.3 to 10 partsby weight, and is preferably 1 to 5 parts by weight, with respect to 100parts by weight of the ink.

D. Solvent

An aqueous solvent is used in an aqueous black or color ink. The aqueoussolvent may contain a water-soluble organic solvent, if necessary.Examples of water-soluble organic solvents include ethylene glycol,diethylene glycol, triethylene glycol, tripropylene glycol, glycerin,1,2,4-butanetriol, 1,2,6-hexanetriol, 1,2,5-pentanetriol,1,2-butanediol, 1,3-butanediol, 1,4-butanediol, dimethyl sulfoxide,diacetone alcohol, glycerin monoallyl ether, propylene glycol, butyleneglycol, polyethylene glycol 300, thiodiglycol, N-methyl-2-pyrrolidone,2-pyrrolidone, γ-butyrolactone, 1,3-dimethyl-2-imidazolididone,sulfolane, trimethylolpropane, trimethylolethane, neopentyl glycol,ethylene glycol monomethyl ether, ethylene glycol monoethyl ether,ethylene glycol monoisopropyl ether, ethylene glycol monoallyl ether,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,triethylene glycol monbmethyl ether, triethylene glycol monoethyl ether,propylene glycol monomethyl ether, dipropylene glycol monomethyl ether,β-dihydroxyethylurea, urea, acetonylacetone, pentaerythritol, and1,3-cyclohexanediol. These solvents may be used in combination.

Further examples of solvents include hexylene glycol, ethylene glycolmonopropyl ether, ethylene glycol monobutyl ether, ethylene glycolmonoisobutyl ether, ethylene glycol monophenyl ether, diethylene glycoldiethyl ether, diethylene glycol monobutyl ether, diethylene glycolmonoisobutyl ether, triethylene glycol monobutyl ether, triethyleneglycol dimethyl ether, triethylene glycol diethyl ether, tetraethyleneglycol dimethyl ether, tetraethylene glycol diethyl ether, propyleneglycol monobutyl ether, dipropylene glycol monomethyl ether, dipropyleneglycol monopropyl ether, dipropylene glycol monobutyl ether,tripropylene glycol monomethyl ether, glycerin monoacetate, glycerindiacetate, glycerin triacetate, ethylene glycol monomethyl acetate,diethylene glycol monomethyl ether acetate, cyclohexanol,1,2-cyclohexanediol, 1-butanol, 3-methyl-1,5-pentanediol,3-hexene-2,5-diol, 2,3-butanediol, 1,5-pentanediol, 2,4-pentanediol, and2,5-hexanediol. Monovalent alcohols may be used in combination to adjustpenetrability and wettability. Examples of the monovalent alcoholinclude ethanol, 1-propanol, 2-propanol, 1-methoxy-2-propanol, furfurylalcohol, and tetrahydrofuran.

The aqueous solvents may be added in a total amount of 5 to 40 percentby weight of the ink.

In the aqueous ink in accordance with the present invention, thepolymerizable oligomer functions not only as an inhibitor of waterevaporation, but also as a solvent, thus, when the ink contains a highcontent of the oligomer, it is preferable to avoid using theaforementioned water-soluble organic solvent.

E. Ink Formulation

The content of each component in the ink will now be described. The dyecontent in the ink is in the range of 0.5 to 7.0 percent by weight. Thepigment content in the ink is in the range of 0.5 to 10 percent byweight. In recent years, inks having low coloring agent contents, thatis, thin inks, have been frequently used to enhance reproducibility ofhalf-tone images. In such a case, the dye or pigment content may be inthe range of 0.1 to 1.0 percent by weight.

Method for Making Ink

In the production of ink, raw materials may be mixed in any order.Preferably, mixing of the above ingredients is performed rapidly so thata nonhomogeneous state does not continue for a long time. When a pigmentdispersion is used, vigorous agitation is preferable to achieve ahomogeneous mixing state.

Ink Characteristics

The ink in accordance with the present invention is preferably used in arecording method which discharges ink droplets by applying mechanicalenergy to the ink, and a recording method which discharges ink dropletsby bubbling of the ink by applying thermal energy to the ink. When theink is used in ink-jet recording, it is preferable that the ink haveprecise discharging characteristics from an ink-jet head. In order toproduce a high-quality image on a recording medium, it is preferablethat the ink characteristics be adjusted to maintain a dotted shapehaving a desired size on the recording medium. In view of dischargingcharacteristics from the ink-jet head and ink behavior immediately afterdischarge onto a recording medium, the ink preferably has a viscosity of1 to 15 cps, and a surface tension of 25 dyne/cm or more. Morepreferably, the ink has a viscosity of 1 to 5 cps, and a surface tensionof 25 to 50 dyne/cm.

As the most noticeable feature in the present invention, highlywater-soluble materials, such as a polymerizable oligomer and aphotopolymerization initiator, are used. Since an organic solvent fordissolving the oligomer and the initiator is not added to the ink, theink has superior ink-jet characteristics regardless of addition of theoligomer and the initiator.

Ink Set (Combination of Anionic Ink and Cationic Ink)

When a black ink and a color ink are used in combination as a set ofinks to form a multicolor image, it is preferable that the black ink andthe color ink have polarities opposite to each other. At the boundary ofthe black ink and the color ink on a recording medium, the anioniccomponent and the cationic component react with each other beforephotocuring of the inks. Thus, bleeding between the uncured inks can beeffectively prevented by the synergism of the photocuring and the ionicreaction.

Printer System

With reference to FIG. 1, a printer system which is preferably used inthe present invention includes a conventional ink-jet printing mechanism2 and a light exposure unit 5 such as an ultraviolet exposure unit. Theultraviolet exposure unit 5 irradiates the printed face of recordingpaper 4 which is introduced from a feeder 3 and expelled from a printingstation 1. The best timing of the irradiation is in a state in which theink components are uniform before the ink completely penetrates into therecording paper. Practical printing operations, however, are performedusing various types of recording paper having different penetrationcharacteristics. Thus, it is preferable that the ultraviolet lamp putsnear the print head as close as possible. A preferable ultraviolet lampis a low-pressure mercury lamp having a vapor pressure of mercury of 1to 10 Pa when turned on, or a lamp in which are gas is sealed. Theemission spectra in an ultraviolet region of the low-pressure mercurylamp have a maximum in the range of 184 nm to 450 nm which is suitablefor effective polymerization of the components in the black and colorinks. Furthermore, this lamp can work by a compact electrical powersource. Examples of other ultraviolet lamps used in the presentinvention include a metal halide lamp, a high-pressure mercury lamp, anultrahigh-pressure mercury lamp, a lamp in which are-earth element isused a xenon flash lamp, and a deep UV lamp. These lamps also havesubstantially the same emission spectra. The intensity of theultraviolet light is preferably in the region of 10 to 50 W/cm² tocontrol the polymerization rate within a desired range, although it canbe selected in response to the sensitivity of the photopolymerizationinitiator. If the intensity is too low, the ink on the recording paperdoes not have sufficient bonding force, and in particular, the color inkdoes not have sufficient water resistance. Curing of the ink on therecording paper by ultraviolet light having an optimized intensityimparts satisfactory fixing characteristics, high smear resistance, andhigh water resistance to the ink. Furthermore, the curing suppressescurling and cockling of the recording paper, and thus facilitateshandling and preservation of the paper.

Ink-Jet Recording Apparatus

FIG. 2 is a longitudinal cross-sectional view of a head of an ink-jetrecording apparatus which is used in the printer system shown in FIG. 1,and FIG. 3 is a cross-sectional view taken along line III—III in FIG. 2.A head 13 is formed by bonding a plate provided with channels or tonozzles for conducting ink to a heating element 15. The plate may becomposed of glass, ceramic, silicon, or plastic. The heating element 15includes a protective layer 16 composed of silicon dioxide, siliconnitride, or silicon carbide, electrodes 17 a and 17 b composed ofaluminum, gold, or an aluminum-copper alloy, a thermistor layer 18composed of a high-melting point material such as HfB₂, TaN, or TaAl, aheat accumulating layer 19 composed of thermally oxidized silicon oraluminum oxide, and a substrate 20 composed of a material having highheat dissipating characteristics, such as silicon, aluminum, or aluminumnitride.

When an electrical pulse signal is applied between the electrodes 17 aand 17 b, the region n of the heating element 15 is rapidly heated. Abubble forms in the ink 21 which comes into contact with the region n inthe nozzle 14 and causes expansion of the ink. The meniscus 23 isthereby protruded and the ink is discharged as droplets 24 through adischarging orifice 22 towards a recording medium 25.

FIG. 4 is an outline view of a multihead which consists of a pluralityof heads, as shown in FIG. 2. The multihead is formed by bonding a glassplate 27 having a plurality of nozzles 26 and a thermistor layer 28similar to that in FIG. 2.

FIG. 5 is an isometric outline view of an ink-jet recording apparatusprovided with one of the above heads. One end of a wiping blade 61 issupported by a blade-supporting member like a cantilever, at a homeposition adjacent to the recording region by a recording head 65. Inthis embodiment, the blade 61 is protruded from the path of movement ofthe recording head 65. The recording head 65 is A provided with a cap 62which moves in the direction perpendicular to the moving direction ofthe recording head 65 and comes into contact with the ink-dischargingface. An ink absorber 63 is provided adjacent to the blade 61 and isprotruded from the path of movement of the recording head 65. The blade61, the cap 62, and the ink absorber 63 constitute a discharge recoverysection 64 which removes liquids and dust from the ink-discharging face.

The recording head 65 discharges ink on a recording medium facing theink-discharging face by thermal energy applied to the recording head 65.The recording head 65 is loaded on a carriage 66. The carriage 66 issupported by a guide axis 67 so as to slide along the guide axis 67, andis connected to a belt 69 driven by a motor 68 (the connection is notshown in the drawing). Thus, the recording head 65 can move between therecording region and the vicinity thereof.

A recording medium is fed from a feeding section 51 so as to face theink-discharging face of the recording head 65 by a feeding roller 52,and expelled to an expelling section by expelling rollers 53 driven by amotor (not shown in the drawing) after recording. When the recordinghead 65 returns to the home position after completion of recording, thecap 62 of the discharge recovery section 64 is withdrawn from the pathof movement of the recording head 65 whereas the blade 61 is protrudedfrom the path of movement to wipe the ink-discharging face of therecording head 65. Then, the cap 62 moves towards the path of movementto cover the ink-discharging face of the recording head 65. When therecording head 65 shifts from the home position, the cap 62 is withdrawnfrom the path of movement whereas the blade 61 is protruded from thepath of movement. Thus, the blade 61 wipes the ink-discharging face ofthe recording head 65 again.

The recording head 65 preferably returns to the home position for wipingin a given time interval during the recording operation, in addition tothe discharge recovery operation and the completion of the recordingoperation.

Ink Cartridge

FIG. 6 is a longitudinal cross-sectional view of an embodiment of an inkcartridge 45. The ink cartridge 45 has an ink reservoir 40, such as anink bag, and the end of the ink reservoir 40 is capped with a rubberstopper 42. A needle (not shown in the drawing) is inserted in thestopper 42 when the ink in the ink bag 40 is fed to the recording head.Waste ink in the ink cartridge 45 is absorbed by an ink absorber 44.Preferably, the face of the ink reservoir 40 in contact with the ink isformed of polyethylene.

Recording Unit

In the ink-jet recording apparatus used in the present invention, thehead and the ink cartridge may be separated from each other, asdescribed above, or may be integrated, as shown in FIG. 7. Withreference to FIG. 7, a recording unit 70 has an ink reserving section,such as an ink absorber. Ink in the ink absorber is discharged as inkdroplets from a head section 71 having a plurality of orifices. In thepresent invention, a preferable material for the ink absorber ispolyurethane.

The ink reserving section may be an ink bag containing a spring or thelike, in place of the ink absorber. The recording unit 70 has a hole 72for introducing outside air to the interior. The recording unit 70 canbe substituted for the recording head 65, and thus has a configurationwhich is detachable from the carriage 66.

FIG. 8 is an isometric outline view of another ink-jet recording head.The ink-jet recording head has a substrate having a plurality of nozzlesand a pressure-generating element composed of a piezoelectric materialand a conductive material facing the nozzles, and contains inksurrounding the pressure-generating element. The pressure-generatingelement generates pressure by a voltage applied to it and discharges inkdroplets through the nozzles. Such a recording head is called anon-demand ink-jet recording head.

The recording head includes an ink channel 80 connected to an inkchamber (not shown in the drawing), an orifice plate 81 for dischargingink droplets having a desired volume, a vibration plate 82 for applyingpressure directly to ink, a piezoelectric element 83 connected to thevibration plate 82 and vibrating by an electrical signal, and asubstrate 84 for supporting the orifice plate 81 and the vibration plate82.

The ink channel 80 is formed of, for example, a photosensitive resin.The orifice plate 81 is formed of stainless steel or nickel, and has anozzle 85 which is formed by electroforming or press perforation. Thevibration plate 82 is formed of a metal film such as stainless steel,nickel, or titanium, or a highly resilient resin film. The piezoelectricelement 83 is formed of a dielectric material such as barium titanate orlead zirconium titanate (PZT).

When a pulse voltage is applied to the piezoelectric device 83, thepiezoelectric device 83 produces vibration which causes the deformationof the vibration plate 82 connected to the piezoelectric element 83. Thevibration plate 82 applies a pressure in a direction perpendicular tothe ink in the ink channel 80 to discharge the ink as droplets (notshown in the drawing) through the nozzle 85. This recording head canalso be mounted in a recording apparatus, as shown in FIG. 5.

When a color image is recorded using the above-mentioned ink set, arecording apparatus having four recording heads, of the type shown inFIG. 10, arranged in series on a carriage may be used. FIG. 10 is anisometric outline view of a configuration of four recording headsarranged on a carriage. The recording apparatus has a black recordingunit 91, a cyan recording unit 92, a magenta recording unit 93, and ayellow recording unit 94. These recording units are arranged on acarriage and discharge color inks in response to recording signals.Although four recording units are used in FIG. 10, four color inks maybe independently discharged from one recording head 94, as shown in FIG.9, having four ink cartridges 86 to 89 and corresponding dischargingnozzles.

EXAMPLES

The present invention will now be described in more detail withreference to EXAMPLES.

Synthesis of Photopolymerization Initiator

A photopolymerization initiator represented by the formula (1) wasprepared. Into an autoclave, 5 moles (1121.5 g) of a thoroughly driedpowder chemical (trade name: Irgacure 2959 made by Ciba Specialty Co.)represented by the following formula was placed. Sodium hydroxide as acatalyst was added thereto and the autoclave was heated to 105° C.

While stirring the contents in the autoclave, 10 moles (440 g) ofethylene oxide was fed at a rate of 5 ml/min into the autoclave andallowed to react at 105° C. for 5 hours. After the completion of thereaction, the solution was neutralized with an aqueous hydrochloric acidsolution. The salt was removed through a dialysis membrane, and then thesolution was dehydrated. FIG. 11 is a liquid chromatogram of Irgacure2959 as the starting material, and FIG. 12 is a liquid chromatogram ofthe reaction products of Irgacure 2959 with ethylene oxide. The startingmaterial has a retention time of 11 minutes (peak A in FIG. 11). Thechromatogram shown in FIG. 12 does not have a peak A and suggests thethat Irgacure 2959 reacted completely. Furthermore, the chromatogram inFIG. 12 has a peak B at 12.6 minutes corresponding to a 2-mole ethyleneoxide adduct of Irgacure 2959 (the sum of m and n in formula (1) is 2),a peak C at 14.5 minutes corresponding to a 4-mole adduct, and peaks Dat 16 to 17 minutes corresponding to higher adducts. These products wereisolated by a preparative liquid chromatograph.

Example 1

Black, magenta, cyan, and yellow inks were prepared according to theformulations shown in Tables 1 to 6.

TABLE 1 Cationic Black Pigment Ink (BK-1) Ingredient Content (parts byweight) IJX-55 (Cationic carbon black 6 made by Cabot Corp.) Cationicoligomer 7 8 Photopolymerization initiator 1 1 Diethylene glycol 10Water 75

TABLE 2 Cationic Black Pigment Ink (BK-2) Ingredient Content (parts byweight) IJX-55 (Cationic carbon black 6 made by Cabot Corp.) Cationicoligomer 6 15 Photopolymerization initiator 1 1 Ethylene glycol 10 Water68

TABLE 3 Anionic Magenta Pigment Ink (M-1) Ingredient Content (parts byweight) Dispersion of Pigment Red 122 *1 4 Nonionic oligomer 1 10Photopolymerization initiator 3 2 Thiodiglycol 7 Water 77 Note *1: Ratioof pigment to polymer dispersant = 10:2 (by weight). Dispersant:styrene-acrylic acid-ethyl acrylate copolymer (number average molecularweight: 9,500, acid value: 190).

TABLE 4 Anionic Cyan Pigment Ink (C-1) Ingredient Content (parts byweight) Dispersion of Pigment Blue 15:32 *2 5 Nonionic oligomer 1 12.5Photopolymerization initiator 3 1.5 Polyethylene glycol 300 5Thiodiglycol 5 Water 71 Note *2: Ratio of pigment to polymer dispersant= 10:3 (by weight). Dispersant: styrene-acrylic acid-ethyl acrylatecopolymer (number average molecular weight: 9,800, acid value: 190).

TABLE 5 Anionic Yellow Pigment Ink (Y-1) Ingredient Content (parts byweight) Dispersion of Pigment Yellow 128 *3 5 Nonionic oligomer 1 12Photopolymerization initiator 3 1 Polyethylene Glycol 300 10Thiodiglycol 5 Water 67 Note *3: Ratio of pigment to polymer dispersant= 10:3.5 (by weight). Dispersant: styrene-acrylic acid-ethyl acrylatecopolymer (number average molecular weight: 9,800, acid value: 190).

TABLE 6 Cationic Black Pigment Ink (BK-4) Ingredient Content (parts byweight) Dispersion of Color Black No. 960 *4 6 Cationic oligomer 8 15Photopolymerization initiator 4 1 Glycerin 10 Ethylene glycol 5 Water 63Note *4: Carbon black dispersion (average particle size: 115 nm) using acopolymer of acrylonitrile, trimethylamino-2-hydroxypropyl methacrylateand acrylamide (monomer ratio by weight = 20:60:20, number averagemolecular weight: 4,500) as a dispersant. The ratio of the pigment tothe dispersant = 10:2.5 (by weight).

Example 2

Using the inks prepared in EXAMPLE 1, images were recorded on plainpaper (trade name: plain paper NSK for electrophotographic copy, made byCanon Kabushiki Kaisha) by a bubble-jet printer BJC-430 (made by CanonKabushiki Kaisha) provided with an ultraviolet lamp, as shown in FIG. 1.The lamp was a low-pressure mercury lamp UL1-5DQ (input power: 50 W, 90V, 0.6 A) made by Ushio Inc. The time from ink discharge to ultravioletexposure was approximately 7 seconds, and the exposure time wasapproximately 5 seconds. An image including black characters and solidcolor patterns was printed and exposed to ultraviolet light. The printedblack letters were clear without bleeding into the solid color patterns.The prints were evaluated as follows. The results are shown in Table 7,with the results of prints not exposed to ultraviolet light forcomparison.

TABLE 7 Wet smear Dry smear Water Marker Ink UV exposure resistanceresistance resistance resistance BK-1 Exposed A A A A Not exposed B B BB BK-2 Exposed A A A A Not exposed B B B B M-1 Exposed A A A A Notexposed B B B B C-1 Exposed A A A A Not exposed B B B B Y-1 Exposed A AA A Not exposed B B B B BK-4 Exposed A A A A Not exposed B B B B

Testing Methods

1. Wet Smear Resistance

The printed characters were rubbed with wet tissue paper and bleedingwas evaluated on the basis of the following standards:

A: No bleeding was seen.

B: Contamination was found at the periphery of the rubbed section.

C: Significant contamination was found at the periphery.

2. Dry Smear Resistance

The printed characters were rubbed with an eraser and were evaluated onthe basis of the following standards:

A: Density of the characters did not change.

B: Density decreased and the eraser was contaminated.

C: Density noticeably decreased.

3. Water Resistance

Water droplets were dropped on the printed paper, the paper was tilted,and blurring was evaluated on the basis of the following standards:

A: No blurring of the unprinted section was found.

B: Blurring was found at the periphery of the section where water waddropped.

C: The characters became unclear and the unprinted section wascontaminated.

4. Marker Pen Resistance

The printed characters were overwritten two times with a yellow markerand evaluated on the basis of the following standards:

A: Blurring was not found at a position overwritten later and the markerwas not contaminated.

B: Blurring was found at positions overwritten later and the marker wascontaminated.

C: Significant blurring was found at positions overwritten later and themarker was significantly contaminated.

Example 3

An ink set including anionic dye color inks was prepared according tothe formulations shown in Table 8 to 10.

TABLE 8 Magenta Ink (M-5) Ingredient Content (parts by weight) Magentadye *1 3.5 Nonionic oligomer 3 10 Photopolymerization initiator 3 0.512-mole ethylene oxide adduct of 5 glycerin Thiodiglycol 2 Water 79Note*1: The magenta dye has the following structure:

TABLE 9 Cyan Ink (C-5) Ingredient Content (parts by weight) Direct Blue199 (made by Zeneca 5 Inc.) Nonionic oligomer 3 10 Photopolymerizationinitiator 3 0.5 12-mole ethylene oxide adduct of 10 glycerinThiodiglycol 5 Water 69.5

TABLE 10 Yellow Ink (Y-5) Ingredient Content (parts by weight) DirectYellow 132 (made by Daiwa 5 Dyestuff Mfg. Co., Ltd.) Nonionic oligomer 310 Photopolymerization initiator 3 0.5 12-mole ethylene oxide adduct of10 glycerin Thiodiglycol 5 Water 69.5

Example 4

Using the color inks prepared in EXAMPLE 3 and the black ink BK-1prepared in EXAMPLE 1, images were recorded on various types of paper,that is, plain paper NSK for electrophotographic copy made by CanonKabushiki Kaisha, Neusiedler paper, Paper 4024 made by Fuji Xerox Co.,Ltd., and Gilbert Bond paper, by a bubble-jet printer BJC-430 (made byCanon Kabushiki Kaisha) provided with an ultraviolet lamp, as in EXAMPLE2. An image including black characters and solid color patterns wasprinted and exposed to ultraviolet light. The printed black letters wereclear without bleeding into the solid color patterns. The prints wereevaluated as follows. The results are shown in Table 11, with the waterresistance results for prints not exposed to ultraviolet light forcomparison. The smear resistance was evaluated as in EXAMPLE 2.

Testing Methods

5. Water Resistance

Each recording paper was immersed in tap water for one minute. Theresidual rate of the optical density (OD) with respect to untested paperwas calculated, wherein the residual rate is represented by thefollowing equation:

{(optical density of print immersed in water for 1 minute)/(initialoptical density)}×100

A: The residual rate was in the range of 80% to 120%.

B: The residual rate was in the range of 50% to 80%.

C: The residual rate was in the range of 30% to 50%.

6. Discoloration Rate

The discoloration rate was determined by the following equation:

Discoloration rate=(reflection density of a sample exposed to UVlight/reflection density of a sample not exposed to UV light)×100

TABLE 11 Dis- Smear Resistance Water Resistance color- UV exposed UVunexposed UV ex- UV un- ation Ink Wet Dry Wet Dry posed exposed RateBlack BK-1 A A B B A B 100 Magenta M-5 A A B B A C  86 Cyan C-5 A A B BA C  98 Yellow Y-5 A A B B A C  82 Note: The results in Table 11indicate an average for the four types of paper.

Example 5

An aqueous black ink BK-3 was prepared according to the formulationshown in Table 12. This ink does not contain a water-soluble organicsolvent. The ink was loaded in an ink-jet printer and was used forprinting tests. Problems such as unsatisfactory discharge did not occurduring continuous printing or during secondary printing after a pause,probably due to the function of the oligomer as a humectant.

TABLE 12 Black Ink (BK-3) Ingredient Content (parts by weight) IJX-55(Cationic carbon black 5 made by Cabot Corp.) Cationic oligomer 4 20Photopolymerization initiator 3 2 Water 73

Example 6

An aqueous anionic black ink BK-5 was prepared according to theformulation shown in Table 13. Using an ink set of a black ink BK-1, amagenta ink M-5, a cyan ink C-5, and a yellow ink Y-5, an imageincluding black characters on color solid background patterns wasformed, as in EXAMPLE 2. Furthermore, using an ink set of a black inkBK-5, a magenta ink M-5, a cyan ink C-5, and a yellow ink Y-5, an imageincluding black characters on color solid background patterns wereformed, as in EXAMPLE 2. Bleeding between the black characters and solidcolor patterns of these images were visually evaluated. The results areshown in Table 14.

TABLE 13 Black Ink (BK-5) Ingredient Content (parts by weight) Anioniccarbon black dispersion 25 having a solid content of 15.3% by weight(Cabojet 300 made by Cabot Corp.) Nonionic oligomer 4 10Photopolymerization initiator 3 1 Diethylene glycol 15 Water 49

TABLE 14 Crispness of Bleeding of Ink Combination Black Black Ink toBlack Color Characters Color Portions BK-1 M-5, C-5, Y-5 Crisp None(Cationic) BK-5 M-5, C-5, Y-5 Not crisp Whisker (Anionic) (thickbleeding characters)

As shown in Table 14, a combination of a black ink and a set of colorinks having a polarity opposite to that of the black ink cansatisfactorily prevent bleeding on plain paper.

As described above, a pigment dispersion aqueous black ink in accordancewith the present invention can totally overcome problems in conventionalpigment dispersion aqueous black inks. In particular, this aqueous blackink has high image density, high water and light resistance,satisfactory initial penetration and fixing characteristics, and highadhesiveness to recording media. Furthermore, an ink-jet recordingmethod can prevent bleeding between the black ink and color inks.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. An ink for ink-jet recording comprising acoloring agent, a polymerizable cationic oligomer, a photopolymerizationinitiator and water, the photopolymerization initiator having asolubility in water of 3 percent by weight or more, wherein the cationicoligomer has a basic structure selected from the following formulae (6)to (17):

wherein R is a polyol moiety, X is hydrogen or CH₃, and A⁻ is Cl⁻, HSO₃⁻ or CH₃COO⁻.
 2. An ink for ink-jet recording comprising a coloringagent, a polymerizable cationic oligomer, a photopolymerizationinitiator and water, the polymerizable oligomer having at least twoacryloyl groups and a solubility in water of 10 percent by weight ormore, wherein the cationic oligomer has a basic structure selected fromthe following formulae (6) to (17):

wherein R is a polyol moiety, X is hydrogen or CH₃, and A⁻ is Cl⁻, HSO₃^(− or CH) ₃COO⁻.
 3. An ink according to claim 2, wherein thephotopolymerization initiator has a solubility in water of 3 percent byweight or more.
 4. An ink according to any one of claims 1 to 3, whereinthe coloring agent is a pigment.
 5. An ink according to claim 4, whereinthe pigment is a self-dispersing carbon black having surface-bondedcationic groups.
 6. An ink according to claim 5, wherein the cationicgroup is an ammonium group or a quaternary ammonium salt of a tertiaryamine.
 7. An ink according to claim 5, wherein the photopolymerizationinitiator is a cationic photopolymerization initiator.
 8. An inkaccording to claim 5, wherein the cationic oligomer is selected fromcompounds represented by the following formulae (18) to (28):

wherein n+m+k=15;


9. An ink according to claim 4, further comprising a polymeric compoundhaving cationic groups as a dispersant for dispersing the pigment in thewater.
 10. An ink according to claim 9, wherein the photopolymerizationinitiator is a cationic photopolymerization initiator.
 11. An inkaccording to claim 9, wherein the cationic oligomer is selected fromcompounds represented by the following formulae (18) to (28):

wherein n+m+k=15;


12. An ink according to any one of claims 1 to 3, wherein the coloringagent is a cationic dye.
 13. An ink according to claim 12, wherein thecationic dye is at least one dye selected from the group consisting ofC.I. Basic Yellow 1, C.I. Basic Yellow 11, C.I. Basic Yellow 13, C.I.Basic Yellow 19, C.I. Basic Yellow 21, C.I. Basic Yellow 25, C.I. BasicYellow 33, C.I. Basic Yellow 36, C.I. Basic Red 1, C.I. Basic Red 2,C.I. Basic Red 9, C.I. Basic Red 12, C.I. Basic Red 13, C.I. Basic Red38, C.I. Basic Red 39, C.I. Basic Red 92, C.I. Basic Blue 1, C.I. BasicBlue 3, C.I. Basic Blue 5, C.I. Basic Blue 9, C.I. Basic Blue 19, C.I.Basic Blue 24, C.I. Basic Blue 25, C.I. Basic Blue 26, C.I. Basic Blue28, C.I. Basic Blue 45, C.I Basic Blue 54, and C.I. Basic Blue
 65. 14.An ink according to claim 12, wherein the photopolymerization initiatoris a cationic photopolymerization initiator.
 15. An ink according toclaim 8, wherein the cationic oligomer is selected from compoundsrepresented by the following formulae (18) to (28):

wherein n+m+k=15;


16. An ink according to any one of claims 1 to 3, wherein thephotopolymerization initiator is a nonionic photopolymerizationinitiator.
 17. An ink according to claim 16, wherein the nonionicphotopolymerization initiator is a compound represented by the generalformula (1):

wherein n is an integer of 1 or more, m is an integer of 0, or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.
 18. An ink according to claim 17, wherein the compoundis at least one compound represented by the formulae (29) to (33):


19. A method for forming an image on a recording medium comprising thesteps of: (a) imparting an ink according to claim 1 or 2 on the surfaceof the recording medium so as to form an image; and (b) irradiating theink on the recording medium with light.
 20. A method for forming animage according to claim 19, wherein the coloring agent is a pigment.21. A method for forming an image according to claim 20, wherein thepigment is a self-dispersing carbon black having surface-bonded cationicgroups.
 22. A method for forming an image according to claim 21, whereinthe photopolymerization initiator is a cationic photopolymerizationinitiator.
 23. A method for forming an image according to claim 21,wherein the cationic oligomer is selected from compounds represented bythe following formulae (18) to (28):

wherein

represents cyclohexane;

wherein n+m+k=15;


24. A method for forming an image according to claim 23, wherein thecationic group is an ammonium group or a quaternary ammonium salt of atertiary amine.
 25. A method for forming an image according to claim 20,wherein said ink further comprises a polymeric compound having cationicgroups as a dispersant for dispersing the pigment in the water.
 26. Amethod for forming an image according to claim 25, wherein thephotopolymerization initiator is a cationic photopolymerizationinitiator.
 27. A method for forming an image according to claim 25,wherein the cationic oligomer is selected from compounds represented bythe following formulae (18) to (28):

wherein n+m+k 15;


28. A method for forming an image according to claim 19, wherein thecoloring agent is a cationic dye.
 29. A method for forming an imageaccording to claim 28, wherein the photopolymerization initiator is acationic photopolymerization initiator.
 30. A method for forming animage according to claim 28, wherein the cationic oligomer is selectedfrom compounds represented by the following formulae (18) to (28):

wherein n+m+k=15;


31. A method for forming an image according to claim 19, wherein thephotopolymerization initiator is a nonionic photopolymerizationinitiator.
 32. A method for forming an image according to claim 31,wherein the nonionic photopolymerization initiator is a compoundrepresented by the general formula (1):

wherein n is an integer of 1 or more, m is an integer of 0 or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.
 33. A method for forming an image according to claim 32,wherein the compound is at least one compound represented by theformulae (29) to (33):


34. A method for forming an image according to claim 19, wherein thestep (a) is performed by an ink-jet process.
 35. A method for forming animage according to claim 34, wherein the ink-jet process comprises astep for discharging the ink from an orifice by thermal energy impartedto the ink.
 36. A method for forming an image according to claim 34,wherein the ink-jet process comprises a step for discharging the inkfrom an orifice by mechanical energy imparted to the ink.
 37. A methodfor forming an image according to claim 19, wherein the light isultraviolet light.
 38. A method for recording a multicolor imageincluding a black image by imparting a black ink and at least one colorink selected from a yellow ink, a magenta ink, and a cyan ink to arecording medium by an ink-jet process so as to form an image,comprising the steps of: (a) imparting a black ink having any one of thefollowing Compositions (i) to (iv) to the recording medium by an ink-jetprocess: Composition (i); at least one coloring agent selected from thegroup consisting of a combination of a black pigment and a water-solublecationic polymeric compound for dispersing the black pigment in water, acationic black dye, and a cationic self-dispersing carbon black; acationic polymerizable oligomer; a nonionic or cationicphotopolymerization initiator having a solubility in water of 3 percentby weight or more; and water; Composition (ii): at least one coloringagent selected from the group consisting of a combination of a blackpigment and a water-soluble cationic polymeric compound for dispersingthe black pigment in water, a cationic black dye, and a cationicself-dispersing carbon black; a cationic polymerizable oligomer havingat least two acryloyl groups and having a solubility in water of 10percent by weight or more; a nonionic or cationic photopolymerizationinitiator; and water; Composition (iii): at least one coloring agentselected from the group consisting of a combination of a black pigmentand a water-soluble anionic polymeric compound for dispersing the blackpigment in water, an anionic black dye, and an anionic self-dispersingcarbon black; a nonionic polymerizable oligomer; a nonionicphotopolymerization initiator having a solubility in water of 3 percentby weight or more; and water; Composition (iv): at least one coloringagent selected from the group consisting of a combination of a blackpigment and a water-soluble anionic polymeric compound for dispersingthe black pigment in water, an anionic black dye, and an anionicself-dispersing carbon black; a nonionic polymerizable oligomer havingat least two acryloyl groups and having a solubility in water of 10percent by weight or more; a nonionic photopolymerization initiator; andwater; (b) imparting a color ink having any one of the followingCompositions (v) to (viii) to the recording medium by an ink-jetprocess: Composition (v): at least one coloring agent selected from thegroup consisting of a combination of a color pigment and a water-solublecationic polymeric compound for dispersing the color pigment in water,and a cationic color dye; a cationic polymerizable oligomer; a nonionicor cationic photopolymerization initiator having a solubility in waterof 3 percent by weight or more; and water; Composition (vi): at leastone coloring agent selected from the group consisting of a combinationof a color pigment and a water-soluble cationic polymeric compound fordispersing the color pigment in water, and a cationic color dye; acationic polymerizable oligomer having at least two acryloyl groups andhaving a solubility in water of 10 percent by weight or more; a nonionicor cationic photopolymerization initiator; and water; Composition (vii):at least one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble anionic polymericcompound for dispersing the color pigment in water, and an anionic colordye; a nonionic polymerizable oligomer; a nonionic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more;and water; Composition (viii): at least one coloring agent selected fromthe group consisting of a combination of a color pigment and awater-soluble anionic polymeric compound for dispersing the colorpigment in water, and an anionic color dye; a nonionic polymerizableoligomer having at least two acryloyl groups and having a solubility inwater of 10 percent by weight or more; a nonionic photopolymerizationinitiator; and water; and (c) irradiating the black ink and the colorink imparted to the recording medium with light for photo-crosslinkingof the inks; wherein the black ink and the color ink are selected sothat the polarity of the black ink is opposite to the polarity of thecolor ink, and the cationic polymerizable oligomer has a basic structureselected from the following formulae (6) to (17):

wherein R is a polyol moiety, X is hydrogen or CH₃, and A⁻ is Cl⁻, HSO₃⁻ or CH₃COO⁻.
 39. An ink for ink-jet recording comprising a coloringagent, a polymerizable oligomer, a nonionic photopolymerizationinitiator and water, wherein the nonionic photopolymerization initiatoris a compound represented by the general formula (1):

wherein n is an integer of 1 or more, m is an integer of 0 or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.
 40. An ink according to claim 39, wherein the compoundis at least one compound represented by the formulae (29) to (33):


41. An ink according to claim 39, wherein the polymerizable oligomer isa nonionic oligomer.
 42. An ink according to claim 41, wherein thenonionic oligomer is selected from compounds represented by thefollowing formulae (2) to (5):


43. An ink according to claim 41 or 39, wherein the coloring agent is apigment.
 44. An ink according to claim 43, wherein the pigment is aself-dispersing carbon black having surface-bonded cationic groups. 45.An ink according to claim 44, wherein the cationic group is an ammoniumgroup or a quaternary ammonium salt of a tertiary amine.
 46. An inkaccording to claim 43, further comprising a polymeric compound havingcationic groups as a dispersant for dispersing the pigment in the water.47. An ink according to claim 41 or 39, wherein the coloring agent is acationic dye.
 48. An ink according to claim 47, wherein the cationic dyeis at least one dye selected from the group consisting of C.I. BasicYellow 1, C.I. Basic Yellow 11, C.I. Basic Yellow 13, C.I. Basic Yellow19, C.I. Basic Yellow 21, C.I. Basic Yellow 25, C.I. Basic Yellow 33,C.I. Basic Yellow 36, C.I. Basic Red 1, C.I. Basic Red 2, C.I. Basic Red9, C.I. Basic Red 12, C.I. Basic Red 13, C.I. Basic Red 38, C.I. BasicRed 39, C.I. Basic Red 92, C.I. Basic Blue 1, C.I. Basic Blue 3, C.I.Basic Blue 5, C.I. 1, Basic Blue 9, C.I. Basic Blue 19, C.I. Basic Blue24, C.I. Basic Blue 25, C.I. Basic Blue 26, C.I. Basic Blue 28, C.I.Basic Blue 45, C.I. Basic Blue 54, and C.I. Basic Blue
 65. 49. An inkaccording to claim 43, wherein the pigment is a self-dispersing carbonblack having surface-bonded anionic groups.
 50. An ink according toclaim 49, wherein the anionic group is at least one group selected fromthe group consisting of —COOM, —SO₃M, —PO₃HM, and —PO₃M₂, wherein M is ahydrogen atom, an alkaline metal, an ammonium ion, or an organicammonium group.
 51. An ink according to claim 43, further comprising apolymeric compound having anionic groups as a dispersant for dispersingthe pigment in the water.
 52. An ink according to claim 41 or 39,wherein the coloring agent is an anionic dye.
 53. An ink according toclaim 52, wherein the anionic dye is at least one yellow dye selectedfrom the group consisting of Acid Yellow 11, Acid Yellow 17, Acid Yellow23, Acid Yellow 25, Acid Yellow 29, Acid Yellow 42, Acid Yellow 49, AcidYellow 61, Acid Yellow 71, Direct Yellow 12, Direct Yellow 24, DirectYellow 26, Direct Yellow 44, Direct Yellow 86, Direct Yellow 87, DirectYellow 98, Direct Yellow 100, Direct Yellow 130, Direct Yellow 132,Direct Yellow 142, and dyes represented by the following formulae (34)and (35):

wherein R=H or CH₃, X=H, OH or CH₃, and n=1 or 2;

wherein R=H or CH₃, X=H, OH or CH₃, —L—=—CH₂CH₂—,

and n=1 or
 2. 54. An ink according to claim 52, wherein the anionic dyeis at least one magenta dye selected from the group consisting of AcidRed 1, Acid Red 6, Acid Red 8, Acid Red 32, Acid Red 35, Acid Red 37,Acid Red 51, Acid Red 52, Acid Red 80, Acid Red 85, Acid Red 87, AcidRed 92, Acid Red 94, Acid Red 115, Acid Red 180, Acid Red 254, Acid Red256, Acid Red 289, Acid Red 315, Acid Red 317, Direct Red 1, Direct Red4, Direct Red 13, Direct Red 17, Direct Red 23, Direct Red 28, DirectRed 31, Direct Red 62, Direct Red 79, Direct Red 81, Direct Red 83,Direct Red 89, Direct Red 227, Direct Red 240, Direct Red 242, DirectRed 243 and dyes represented by the following formulae (36) to (38):

wherein R=H, OH or CH₃, —L—=—CH₂CH₂—,

and n=1 or 2;

wherein R=OH or CH₃, and n=1 or
 2. 55. An ink according to claim 52,wherein the anionic dye is at least one cyan dye selected from the groupconsisting of Acid Blue 9, Acid Blue 22, Acid Blue 40, Acid Blue 59,Acid Blue 93, Acid Blue 102, Acid Blue 104, Acid Blue 113, Acid Blue117, Acid Blue 120, Acid Blue 167, Acid Blue 229, Acid Blue 234, AcidBlue 254, Direct Blue 6, Direct Blue 22, Direct Blue 25, Direct Blue 71,Direct Blue 78, Direct Blue 86, Direct Blue 90, Direct Blue 106, DirectBlue 199, and dyes represented by the following compounds (39) and (40):

wherein x+y=3 or 4, X=H, NH₂ or CH₃, R=H or CH₂, —L—=—CH₂CH₂—,

and n=1 or 2;

wherein x+y=3 or
 4. 56. An ink according to claim 39, wherein thepolymerizable oligomer is a cationic oligomer having a basic structureselected from the following formulae (6) to (17):

wherein R is a polyol moiety, X is hydrogen or CH₃, and A⁻ is Cl⁻, HSO₃⁻ or CH₃COO⁻.
 57. An ink according to claim 56, wherein the coloringagent is a cationic dye.
 58. An ink according to claim 57, wherein thecationic dye is at least one dye selected from the group consisting ofC.I. Basic Yellow 1, C.I. Basic Yellow 11, C.I. Basic Yellow 13, C.I.Basic Yellow 19, C.I. Basic Yellow 21, C.I. Basic Yellow 25, C.I. BasicYellow 33, C.I. Basic Yellow 36, C.I. Basic Red 1, C.I. Basic Red 2,C.I. Basic Red 9, C.I. Basic Red 12, C.I. Basic Red 13, C.I. Basic Red38, C.I. Basic Red 39, C.I. Basic Red 92, C.I. Basic Blue 1, C.I. BasicBlue 3, C.I. Basic Blue 5, C.I. Basic Blue 9, C.I. Basic Blue 19, C.I.Basic Blue 24, C.I. Basic Blue 25, C.I. Basic Blue 26, C.I. Basic Blue28, C.I. Basic Blue 45, C.I. Basic Blue 54, and C.I. Basic Blue
 65. 59.An ink according to claim 56, wherein the cationic oligomer is selectedfrom compounds represented by the following formulae (18) to (28):

wherein n+m+k=15;


60. An ink according to claim 56, wherein the coloring agent is apigment.
 61. An ink according to claim 60, wherein the pigment is aself-dispersing carbon black having surface-bonded cationic groups. 62.An ink according to claim 61, wherein the cationic group is an ammoniumgroup or a quaternary ammonium salt of a tertiary amine.
 63. An inkaccording to claim 60, further comprising a polymeric compound havingcationic groups as a dispersant for dispersing the pigment in the water.64. A method for forming an image on a recording medium comprising thesteps of: (a) imparting an ink according to claim 82 on the surface ofthe recording medium so as to form an image; and (b) irradiating the inkon the recording medium with light.
 65. A method for forming an imageaccording to claim 64, wherein the nonionic photopolymerizationinitiator is at least one compound represented by the formulae (29) to(33):


66. A method for forming an image according to claim 64, wherein thepolymerizable oligomer is a nonionic oligomer.
 67. A method for formingan image according to claim 66, wherein the nonionic oligomer isselected from compounds represented by the following formulae (2) to(5):


68. A method for forming an image according to claim 64 or 66, whereinthe coloring agent is a pigment.
 69. A method for forming an imageaccording to claim 68, wherein the pigment is a self-dispersing carbonblack having surface-bonded cationic groups.
 70. A method for forming animage according to claim 68, wherein said ink further comprises apolymeric compound having cationic groups as a dispersant for dispersingthe pigment in the water.
 71. A method for forming an image according toclaim 64 or 66, wherein the coloring agent is a cationic dye.
 72. Amethod for forming an image according to claim 68, wherein the pigmentis a self-dispersing carbon black having surface-bonded anionic groups.73. A method for forming an image according to claim 68, wherein saidink further comprises a polymeric compound having anionic groups as adispersant for dispersing the pigment in the water.
 74. A method forforming an image according to claim 64 or 66, wherein the coloring agentis an anionic dye.
 75. A method for recording a multicolor imageincluding a black image on a recording medium comprising the steps of:(a) imparting a black ink comprising a coloring agent, a polymerizableoligomer, a photopolymerization initiator and water on the surface ofthe recording medium so as to form an image; (b) imparting a color inkcomprising a coloring agent, a polymerizable oligomer, aphotopolymerization initiator and water on the surface of the recordingmedium so as to form an image; and (c) irradiating the black ink and thecolor ink imparted to the recording medium with light forphotocrosslinking of the inks; wherein the polarity of the black ink isopposite to the polarity of the color ink, and wherein at least one ofthe photopolymerization initiator of the black ink and that of the colorink is a nonionic photopolymerization initiator represented by thegeneral formula (I):

wherein n is an integer of 1 or more, m is an integer of 0 or more, thesum of n and m is in a range of 1 to 8, and n is in a range of 1 to 8when m is zero.
 76. An ink set for ink-jet recording comprising a blackink and a color ink, wherein (a) the black ink has any one of thefollowing compositions (i) to (iv): Composition (i): at least onecoloring agent selected from the group consisting of a combination of ablack pigment and a water-soluble cationic polymeric compound fordispersing the black pigment in water, a cationic black dye, and acationic self-dispersing carbon black; a nonionic or cationicpolymerizable oligomer; a nonionic or cationic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more;and water; Composition (ii): at least one coloring agent selected fromthe group consisting of a combination of a black pigment and awater-soluble cationic polymeric compound for dispersing the blackpigment in water, a cationic black dye, and a cationic self-dispersingcarbon black; a nonionic or cationic polymerizable oligomer having atleast two acryloyl groups and having a solubility in water of 10 percentby weight or more; a nonionic or cationic photopolymerization initiator;and water; Composition (iii): at least one coloring agent selected fromthe group consisting of a combination of a black pigment and awater-soluble anionic polymeric compound for dispersing the blackpigment in water, an anionic black dye, and an anionic self-dispersingcarbon black; a nonionic polymerizable oligomer; a nonionicphotopolymerization initiator having a solubility in water of 3 percentby weight or more; and water; Composition (iv): at least one coloringagent selected from the group consisting of a combination of a blackpigment and a water-soluble anionic polymeric compound for dispersingthe black pigment in water, an anionic black dye, and an anionicself-dispersing carbon black: a nonionic polymerizable oligomer havingat least two acryloyl groups and having a solubility in water of 10percent by weight or more; a nonionic photopolymerization initiator; andwater; (b) the color ink has any one of the following compositions (v)to (viii): Composition (v): at least one coloring agent selected fromthe group consisting of a combination of a color pigment and awater-soluble cationic polymeric compound for dispersing the colorpigment in water, and a cationic color dye; a nonionic or cationicpolymerizable oligomer; a nonionic or cationic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more;and water; Composition (vi): at least one coloring agent selected fromthe group consisting of a combination of a color pigment and awater-soluble cationic polymeric compound for dispersing the colorpigment in water, and a cationic color dye; a nonionic or cationicpolymerizable oligomer having at least two acryloyl groups and having asolubility in water of 10 percent by weight or more; a nonionic orcationic photopolymerization initiator; and water; Composition (vii): atleast one coloring agent selected from the group consisting of acombination of a color pigment and a water-soluble anionic polymericcompound for dispersing the color pigment in water, and an anionic colordye; a nonionic polymerizable oligomer; a nonionic photopolymerizationinitiator having a solubility in water of 3 percent by weight or more;and water; Composition (viii): at least one coloring agent selected fromthe group consisting of a combination of a color pigment and awater-soluble anionic polymeric compound for dispersing the colorpigment in water, and an anionic color dye; a nonionic polymerizableoligomer having at least two acryloyl groups and having a solubility inwater of 10 percent by weight or more; a nonionic photopolymerizationinitiator; and water; and the polarity of the black ink is opposite tothe polarity of the color ink.